Electrical resistor



United States Patent 3,216,955 ELECTRICAL RESISTOR Samuel Ruben, 52Seacord Road, New Rochelle, N.Y. No Drawing. Filed Apr. 23, 1963, Ser.No. 274,904 2 Claims. (Cl. 252-512) This invention relates to a stablerefractory electrically conductive element and specifically to anelectrical resistor.

A general object of the invention is to provide a material suitable foruse as an electrical resistor.

Other objects will be apparent from the disclosure.

In the past there has been a general recognition that molybdenumdisilicide is a useful base material for electrical resistance elements.In my Patent 3,006,865 I have described a refractory compositionsuitable for use as an electrical resistance element, comprising thereaction product of molybdenum disilicide and vanadium pentoxide. In mypending US. patent application Serial No. 144,292, filed October 11,1961, I describe an improvement upon the device covered by myaforementioned patent.

The present invention has to do with a new composition, which forcertain applications such as for electrical resistors, is an improvementover my patent and the pending patent application.

I have found that the composition of molybdenum disilicide and elementalboron provides a superior resistance element.

The art discloses many combinations of molybdenum disilicide and bondingreacting agents such as the borides, silicates and oxides. The use ofthese materials affect the electrical conductivity of the molybdenumdisilicide and the resultant products are in most cases unable towithstand thermal shock.

I have produced an improved electrical resistor element by mixing andgrinding molybdenum disilicide granules :Wlth elemental boron andheating to a temperature in the order of 1100 C. to 1200 C. A metal-likedense material is obtained, capable of sustained applications at hightemperature, such as 1200 C., having good electrical conductivity and apositive resistance temperature coefiicient.

The percent of boron added to the molybdenum disilicide determines thedensity of the combination, its resistivity and temperature coefiicient.The preferred range of boron is from 2% to 20%, the balance beingmolybdenum disilicide. The resistance material composition is preparedby ball milling the molybdenum disilicide and boron particles in acarbide lined mill for several hours. For a typical resistor element, Iball mill a mixture of 90% molybdenum disilicide and 10% boron. Rods.046" by 8.25", are pressed at 30 tons per square inch and have apressed density of 3.75 cc. The grain size of the boron and molybdenumdisilicide particles is 1.5 microns as measured with a Fisher sub-sievesizer. After pressing, the rods are heated in a furnace for 30 minutesat 1100 C. When cool the rods are extremely hard and possess goodtensile strength.

Rods made with 5% boron added to 95% molybdenum disilicide are of aconductive refractory nature and have a resistivity of .0034 ohm cm. at1200 C.; rods made with boron, balance molybdenum disilicide have aresistivity of .0041 ohm cm. at 1200 C., and .00168 ohm cm. at roomtemperature. Continuous operation in oxidizing atmospheres producesnegligible change in resistivity, mechanical strength or appearance.

If desired, terminals may be applied to the rods by a /2 nickel platingon each end. The plated ends may then be placed in a hollowed copper rodand the space between the rod and the hollow copper rod terminal filledwith silver solder. This low resistance type contact "Ice is useful inresistors which are continuously operated over long periods of time andserve to maintain a low contact resistance. The copper rod contacts maybe of sufficient mass to dissipate enough energy to keep the ends cool.

The positive resistance temperature coefiicient characteristic of theresistor of this invention is important and keeps the current fromprogressively increasing to a destructive degree such as occurs withmost semiconductors. If desirable, the positive temperature coefiicientmay be reduced by the addition of tin oxide or other refractoryconductive oxides possessing a negative resistance temperaturecoeificient.

The particle size of the component elements, boron and molybdenumdisilicide, influences the preferred mixture range in terms ofresisitivity and mechanical properties at high temperature operation.

An important characteristic which I have observed is that when the boronand molybdenum disilicide are reacted and bonded together there is smalldimensional change, in the order of 1% with the pressures presentlyutilized. This permits objects having mechanical strength at hightemperature to be produced by powder metallurgy processes.

An examination of the structure of the reaction product indicates thatthe elemental boron combines with some of the molybdenum disilicide andalso that a small amount of the boron oxidizes to boric oxide whichcombines with silicates formed during the heating period.

To insure maxim-um electrical stability and density of a resistorelement, it is heated prior to initial use, by an electric currentdischarged through it at a temperature beyond that at which thematerials had been reacted. For example, a unit reacted at 1200" C. isheated to a temperature of about 1500 C. by discharging a currenttherethrough for a short time. This tends to equalize the resistanceover the length of the unit.

The resistivity of the composition may be increased by the addition ofsuch refractory conductive materials as the nitrides, borides orcarbides of Zr, Ti, Ta, Va or Cr; also by the addition of refractorynon-conductors such as micronized aluminum oxide and the silicates andoxides of zirconium.

As the compositions of this invention lend themselves to pressing intothe desired shape prior to reaction, it is possible to produce circularresistor elements for use in rheostats or potentiometers, in place ofwire wound types which tend to oxidize under overload or when operatedat maximum rated capacity.

I claim:

1. An electrical resistor element consisting essentially of 2% to 20%boron, the balance being substantially all molybdenum disilicide, saidresistor element being the product of heating a pressed mixture of boronand molybdenum disilicide particles in the desired proportion to atemperature of about 1100 to 1200 C. for a time sufficient to causereaction of said particles in the mixture.

2. An electrical resistor element as claimed in claim 1, which containsa small amount of boric oxide.

References Cited by the Examiner UNITED STATES PATENTS 3/49 McKinley26457 5/56 Glaser 252518 XR OTHER REFERENCES Dummer: Fixed Resistors.Pitman & Sons (1956),

1. AN ELECTRICAL RESISTOR ELEMENT CONSISTING ESSENTIALLY OF 2% TO 20%BORON, THE BALANCE BEING SUBSTANTIALLY ALL MOLYBDENUM DISILICIDE, SAIDRESISTOR ELEMENT BEING THE PRODUCT OF HEATING A PRESSED MIXTURE OF BORONAND MOLYBDENUM DISILICIDE PARTICLES IN THE DESIRED PROPORTION TO ATEMPERATURE OF ABOUT 1100* TO 1200*C. FOR A TIME SUFFICIENT TO CAUSEREACTION OF SAID PARTICLES IN THE MIXTURE.